A variety of photometric instruments are used for analyzing radiation transmitted or reflected by samples, in order to ascertain the nature of the sample. Spectrophotometers, in particular, are used to provide a spectrographic analysis, for example to determine the existence or concentration of components in a sample, or to provide a "fingerprint" spectrum characteristic of a sample. A typical application is measurement of dissolution rate of a pharmaceutical tablet in a solution, whereby changes in light transmission through the solution are monitored over time. Another type of instrument is used in liquid chromatography in which a sample solute is injected into a flowing solvent while transmitted radiation is monitored.
A conventional class of spectrophotometers is utilized for measuring optical transmission of liquid samples in a spectral range from infrared to ultraviolet. Usually a sample cell for liquid, also known as a "cuvette", such as described in U.S. Pat. No. 4,886,356 (Paradis), is incorporated into the instrument with the liquid being placed in or pumped through the cuvette chamber. Recently optical fibers have been adopted to carry radiation to a cuvette or probe remote from the instrument. In such case a probe may be designed to be immersed in the liquid as described, for example, in U.S. Pat. No. 5,077,481 (Hoult).
It sometimes is desirable to monitor several samples at once, for example for different tablets dissolving in a number of containers over the same period of time. U.S. Pat. No. 4,431,307 (Suovanieme) teaches arranging liquid containers in a matrix, with each container being self contained with its own optics and detector as a simple photometer. Alternatively, a single instrument with one pair of optical fibers can be used to convey radiation to and from a group of cuvettes or probes where optical switching selects the sample being analyzed, for example as disclosed in U.S. Pat. No. 4,968,148 (Chow et al). Switching for optical fibers generally is directed to rerouting radiation from one fiber (or group of fibers) to another fiber (or group of fibers); examples of this type of switching are disclosed in U.S. Pat. Nos. 5,056,886 (Hoult) and 4,626,065 (Mori).
When a single pair of optical fibers convey radiation continuously to and from a single cuvette or probe, or an array of cuvettes, the fibers may deteriorate significantly by extended exposure to the radiation. This particularly occurs in the UV range where the phenomenon is known as solarization.
U.S. Pat. No. 4,989,932 (Landa et al) discloses a multiplexer which enables light to be sent to and received from samples selectively for analysis. A rotatable barrel containing a pair of angled mirrors is placed axially in a light path. One mirror deflects light transversely to an optical fiber which conveys the light to a sample. Another fiber returns light from the sample to the other mirror which deflects the light back along the original path. The barrel is rotated on the axis to select from different pairs of optical fibers which extend in various radial directions. Although a stepper motor is suggested for rotating the barrel, no mechanism is shown for support or rotation in the optical path. Also, precise alignment is necessary for the rotatable mirrors and fibers relative to the path.
An object of the present invention is to provide a means for optimizing optical coupling between a photometric instrument and one or more external samples through the use of optical fibers, particularly for liquid transmittance samples. Another object is to provide such an instrument with a means for switching among many pairs of optical fibers in a limited space. A further objective is to provide such an instrument with a high degree of repeatability between measurement cycles. Yet another objective is to provide such an instrument with a system of optical fibers so that solarization of any one fiber is substantially reduced.